The invention described herein relates to grids used with fuel assemblies in a nuclear reactor and more particularly to a method for welding the overlapping ends of frame members which peripherally enclose interleaved grid straps forming the grid.
As is well known in the art, a fuel assembly grid consists of multiple interleaved metal straps of egg crate configuration which form cells arranged to receive fuel rods and control rod guide thimbles of a fuel assembly. These grids which are spaced along a fuel assembly length are made to precise tolerances to help assure uniform transfer of heat from each fuel rod to coolant circulated up through the assembly and to provide for the generation of power according to pre-established modes of operation.
A particular problem encountered in manufacturing grids of this type is that to obtain exact dimensions in all of the egg-crate cells, four separate frame straps or plates are placed around the grid perimeter, one on each side, and welded together at their overlapping edges at the grid corners. During the time of welding, distortion of the frame straps often takes place and to a degree sufficient to preclude the insertion of fuel rods therein. As a result, reworking of the grid to bring it within proper dimensions is necessary and if this cannot be done, the grid is scrapped. Different designs of tooling therefore have been used to minimize this problem but a size variation in the corner cells still takes place primarily because of contraction of the welded components when subjected to weld heat. Such variation also exceeds established tolerances thus requiring cutting of the weld, reworking of the grid or scrapping the frame because the corner cells were not of an acceptable size.